Devices and methods for smart sensor application

ABSTRACT

An apparatus comprises a load resistance connectable in series with the electronic sensor to form a series resistance of the load resistance and the internal impedance of the electronic sensor; an excitation circuit configured to apply a predetermined voltage to a circuit element; and a measurement circuit configured to: initiate applying the predetermined voltage to the series resistance and determining the series resistance; initiate applying the predetermined voltage to the load resistance and determining the load resistance; and calculate the internal impedance of the sensor using the determined series resistance and the load resistance, and provide the calculated internal impedance to a user or process.

CLAIM OF PRIORITY

This application is a divisional of U.S. patent application Ser. No.16/584,212, filed on Sep. 26, 2019, which is a divisional of U.S. patentapplication Ser. No. 15/433,862, filed on Feb. 15, 2017, which is acontinuation of CN PCT Application No. PCT/CN2017/070608, filed on Jan.9, 2017, which are hereby incorporated by reference in its entireties.

FIELD OF THE DISCLOSURE

This document relates generally to interface circuits for electronicsensors. Some embodiments relate to test circuits for electronicsensors.

BACKGROUND

Smart sensors are electronic circuits that measure some aspect of theirenvironment and trigger computing resources to perform predefinedfunctions in response to the measurements. Smart sensors are useful inapplication such as implementing an internet of things (IoT). Sometimesthe output of a smart sensor needs to be tailored to the monitoringelectronics operatively coupled to the smart sensors to acquireinformation for the computing resources. The present inventors haverecognized a need for improved interface circuits for smart sensorcircuits.

SUMMARY OF THE DISCLOSURE

This document relates generally to interface circuits for electronicsensors.

Aspect 1 of the present disclosure includes subject matter (such as atest circuit for an electronic sensor) comprising a load resistanceconnectable in series with the electronic sensor to form a seriesresistance of the load resistance and the internal impedance of theelectronic sensor; an excitation circuit configured to apply apredetermined voltage to a circuit element; and a measurement circuitconfigured to: initiate applying the predetermined voltage to the seriesresistance and determining the series resistance; initiate applying thepredetermined voltage to the load resistance and determining the loadresistance; and calculate the internal impedance of the sensor using thedetermined series resistance and the load resistance, and provide thecalculated internal impedance to a user or process.

In aspect 2, the subject matter of aspect 1 optionally includes anexcitation circuit configured to apply a specified electrical signalhaving a signal amplitude less than twenty millivolts (20 mV) to theseries resistance, and the internal impedance of the electronic sensoris less than ten ohms (10Ω).

In aspect 3, the subject matter of one or both of Aspects 1 and 2optionally include the electronic sensor being an electro-chemicalsensor.

Aspect 4 includes subject matter (such as an integrated circuit), or canoptionally be combined with the any combination of Aspects 1-3 toinclude such subject matter, comprising an excitation circuit configuredto apply excitation signals to a sensor circuit, wherein the excitationcircuit includes a configurable first circuit gain stage and aconfigurable second circuit gain stage, wherein in a first gain mode theexcitation circuit generates a first excitation signal from a testsignal using a first signal gain applied by the first circuit gain stageand a second signal gain applied by the second circuit gain stage, andin a second gain mode the excitation circuit generates a secondexcitation signal from the test signal using a third signal gain appliedby the first circuit gain stage and a fourth signal gain applied by thesecond circuit gain stage; and a measurement circuit configured toselectively initiate application of the first excitation signal or thesecond excitation signal to the electronic sensor and calculate theinternal impedance of the sensor

In aspect 5, the subject matter of Aspect 4 optionally includes ameasurement circuit configured to initiate application of the firstexcitation signal to the electronic sensor when the internal impedanceof the sensor has a first internal impedance range and initiateapplication of the second excitation signal to the electronic sensorwhen the internal impedance of the sensor has a second internalimpedance range, wherein the first internal impedance is greater thanthe second internal impedance range.

In aspect 6, the subject matter of one or both of Aspects 4 and 5optionally includes the signal gain of the second gain circuit stagebeing one in the first gain mode, and less than one and greater thanzero in the second gain mode.

In aspect 7, the subject matter of one or any combination of Aspects 4-6optionally includes the signal gain of the first gain circuit stagebeing greater than one in the first gain mode, and less than one andgreater than zero in the second gain mode.

In aspect 8, the subject matter of one or any combination of Aspects 4-7optionally includes a digital-to-analog converter (DAC) circuitconfigured to generate the test signal.

Aspect 9 includes subject matter (such as a test circuit), or canoptionally be combined with one or any combination of Aspects 1-8 toinclude such subject matter, comprising: an adjustable bridge resistanceand a calibration resistance for coupling to an electronic sensor; anexcitation circuit configured to apply an excitation signal to theelectronic sensor, bridge resistance and calibration resistance; and ameasurement circuit configured to: apply a first excitation signal to acalibration resistance and measure a calibration current; apply thefirst excitation signal to a first bridge resistance and measure a firstbridge current; apply a second excitation signal to the first bridgeresistance and measure a second bridge current; apply the secondexcitation signal to the sensor and measure a sensor current; andcalculate the internal impedance of the sensor using the calibrationresistance, the calibration current, the first bridge current, thesecond bridge current, and the sensor current.

In aspect 10, the subject matter of Example 9 optionally includes amultiplexer circuit configured to selectively apply an excitation signalto the calibration resistance, the adjustable bridge resistance, or thesensor.

In aspect 11, the subject matter of one or both of Aspects 9 and 10optionally includes a multiplexer circuit configured to apply anexcitation signal to the calibration resistance, and the measurementcircuit is configured to calculate the calibration resistance using theexcitation signal.

In aspect 12, the subject matter of one or any combination of Aspects9-11 optionally includes a measurement circuit configured to set thebridge resistance to a coarse bridge resistance value prior to theapplying the excitation signal to the bridge resistance.

In aspect 13, the subject matter of one or any combination of Aspects9-12 optionally includes a measurement circuit configured to: apply athird excitation signal to the first bridge resistance and measure athird bridge current, apply the third excitation signal to a secondbridge resistance and measure a fourth bridge current, and calculate theinternal impedance of the sensor using the calibration resistance, thecalibration current, the first bridge current, the second bridgecurrent, the third bridge current, the fourth bridge current, and thesensor current.

In aspect 14, the subject matter of one or any combination of Aspects9-13 optionally includes the electronic sensor being an electrochemicalsensor and the resistance of the electronic sensor is representative ofthe remaining useful life of the electrochemical sensor.

Aspect 15 includes subject matter (such as an apparatus), or canoptionally be combined with the subject matter of one or any combinationof Examples 1-14 to include such subject matter comprising an integratedcircuit. The integrated circuit includes an input to receive anelectrical signal from an electronic sensor, wherein the electricalsignal includes a direct current (DC) offset and a varying signalcomponent; a digital-to-analog converter (DAC) circuit configured tosubtract the DC offset from the input signal; a programmable gainamplifier (PGA) operatively coupled to the DAC circuit, wherein the PGAcircuit is configured to apply signal gain to the varying signalcomponent of the input signal; and a measurement circuit configured togenerate a measure of the varying signal component.

In aspect 16, the subject matter of Aspect 15 optionally includes ameasurement circuit that includes an analog-to-digital converter (ADC)circuit configured to generate a measure of the varying signalcomponent.

In aspect 17, the subject matter of one or both of Aspects 15 and 16optionally includes a measurement circuit including a fast Fouriertransform (FTT) circuit configured to measure a frequency response ofthe varying signal component.

In aspect 18, the subject matter of one or any combination of Aspects15-17 optionally includes an electronic sensor operatively coupled tothe integrated circuit, wherein the electronic sensor is a resistiveelectronic sensor.

In aspect 19, the subject matter of one or any combination of Aspects15-18 optionally includes an electronic sensor being a gas sensor andthe electrical signal from the electronic sensor is proportional to anamount of gas in an atmosphere.

In aspect 20, the subject matter of one or any combination of Aspects15-20 optionally includes a detection circuit, wherein the electronicsensor is an oxygen sensor and the detection circuit is configured togenerate an indication of a lower explosive limit according to themeasure of the varying signal component.

Aspect 21 can include, or can optionally be combined with any portion orcombination of any portions of any one or more of Examples 1-20 toinclude, subject matter that can include means for performing any one ormore of the functions of Aspects 1-20, or a machine-readable mediumincluding instructions that, when performed by a machine, cause themachine to perform any one or more of the functions of Aspects 1-20.

These non-limiting aspects can be combined in any permutation orcombination. This section is intended to provide an overview of subjectmatter of the present patent application. It is not intended to providean exclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the presentpatent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIGS. 1A and 1B are block diagrams representing a test circuit andsensor circuit.

FIGS. 2A and 2B are block diagrams representing another test circuit 204electrically coupled to a sensor circuit.

FIG. 3 is a flow diagram of an example of a method of controlling a testcircuit to measure an internal impedance of an electronic sensor.

FIG. 4 is circuit diagram of portions of another example of a testcircuit electrically coupled to a sensor circuit.

FIG. 5 is a flow diagram of another example of a method of controlling atest circuit to measure an internal impedance of an electronic sensor.

FIG. 6 is circuit diagram of portions of another example of a testcircuit electrically coupled to a sensor circuit.

FIG. 7 is a flow diagram of an example of a method of controlling a testcircuit to measure an internal impedance of an electronic sensor.

FIG. 8 is a circuit diagram of portions of another example of a testcircuit electrically coupled to a sensor circuit.

FIG. 9 is an example of the output of a sensor circuit.

FIG. 10 is a flow diagram of an example of a method of controlling atest circuit to measure an internal impedance of an electronic sensor.

DETAILED DESCRIPTION

Some smart sensor circuits can include electro-chemical sensors tomonitor an amount or concentration of gas. System demands on the sensorsoften require a sensor circuit to have low power consumption and exhibitlow noise to reduce errors. To maximize longevity and minimizeservicing, it is desired for the monitoring circuits to also have lowpower consumption. This can provide challenges in designing circuits tomonitor different types of smart sensor circuits. For instance, it maybe necessary to measure the impedance of a sensor circuit for diagnosticpurposes, but smart sensors can have a wide range of internalimpedances. Some sensor may have an internal resistance greater than tenkilo-ohms (10 kΩ) while other sensors may have a low internal resistance(e.g., 1Ω). A sensor with a low internal impedance typically requires ahigher voltage measurement signal to provide an adequate signal to noiseratio (SNR). However, applying a higher voltage measurement signal to alow impedance can lead to high current consumption by the measurementcircuit and sometimes the sensor circuit cannot tolerate the currentsassociated with the higher measurement.

FIGS. 1A and 1B are block diagrams of a test circuit 104 and sensorcircuit 106. The test circuit 104 may be included on an integratedcircuit. To measure the internal impedance of the sensor circuit, anexcitation signal is applied between a drive or “D” connection and atransimpedance amplifier (TIA) or “T” connection. The “P” and “N”connections are sensing nodes for operation of the sensor. To determinethe internal impedance of the sensor circuit, the test circuit appliesan excitation signal to the sensor circuit using the D and Tconnections. In FIG. 1A the excitation signal has a magnitude of fifteenmillivolts (15 mV), and the sensor circuit 106 has an internal impedanceof 181Ω. Applying the excitation signal results in a current through thesensor of 0.08 milliamps (0.08 mA). The test circuit can determine theinternal resistance of the sensor circuit using Ohm's Law. The testcircuit can provide the determined internal impedance to a user (e.g.,by presenting the value of on a display) or to a process (e.g., aprocess that stores the value in memory or a process that uses thedetermined impedance to take some action).

In FIG. 1B, the sensor circuit 106 has an internal resistance of 1Ω, butit may be desired to measure the sensor with the same 15 mV excitationsignal. However, a straightforward application of the excitation signalresults in a current of 15 mA. This current may cause problems formultiple reasons. For instance, the current may be too large for adesired power consumption, or the sensor circuit may not be able totolerate a current of that magnitude. Also, trying to accommodate acurrent of 0.08 mA and a current of 15 mA may cause accuracy issues inmeasurements made by the monitoring circuits. Reducing the excitationvoltage may significantly reduce the SNR.

FIGS. 2A and 2B are block diagrams of further examples of a test circuit204 electrically coupled to a sensor circuit 206. The test circuit canbe used to determine internal impedances of sensor circuits of about200Ω and also sensor circuits with internal impedances less than 10Ω.The test circuit includes a load resistance R_(load), an excitationcircuit 208, and a measurement circuit 210. The load resistance may havea resistance value of 100Ω (in certain embodiments the load resistancehas a resistance value of about 200Ω). The measurement circuit caninclude logic circuits to perform the described functions. Invariations, the measurement circuit includes a processor such as amicroprocessor. In FIG. 2A, when a sensor circuit with a low impedanceis to be monitored, the measurement circuit 210 connects the loadresistance in series with the internal impedance of the sensor circuitto form a series resistance. The excitation circuit 208 applies anexcitation signal having a predetermined voltage to the seriesresistance. In variations the predetermined voltage is less than orequal to 20 mV. The measurement circuit determines the series resistanceusing the resulting current and predetermined voltage. In FIG. 2B, thetest circuit then applies the excitation signal to just the loadresistance. The excitation signal determines the load resistance usingthe resulting current and predetermined voltage. The test circuitdetermines the internal impedance of the sensor circuit by subtractingthe load resistance from the series resistance.

Adding in the load resistance results in an internal impedance that iscomparable between the two types of sensors. This improves the accuracyin the values of impedance determined by the test circuit. In someexamples, the measurement circuit calibrates the predetermined voltageof the excitation signal. A calibration resistor of a predeterminedresistor can be electrically connected to outputs of the test circuitand the test circuit can adjust the excitation signal until a specifiedcurrent is measured corresponding to the predetermined voltage of theexcitation signal.

FIG. 3 is a flow diagram of an example of a method 300 of controlling atest circuit to measure an internal impedance of an electronic sensor.At 305, a load resistance is electrically connected in series with thesensor to form a series resistance of the load resistance and theinternal impedance of the sensor. At 310, an excitation signal with apredetermined voltage is applied to the series resistance anddetermining the series resistance is determined (e.g., by Ohm's Law). At315, the predetermined voltage is applied to the load resistance and theload resistance is determined. At 320, the internal impedance of thesensor is determined using the determined load resistance and seriesresistance. A value of the internal impedance is provided to a user orprocess. The approach can be used for other ranges of internalimpedance. For example, the test circuit can be used to measure sensorinternal impedances of about 1 kΩ to 1Ω.

FIG. 4 is circuit diagram of portions of an example of a test circuit404 electrically coupled to a resistor (Rx) representing the internalimpedance of a sensor circuit 406. The test circuit 404 may be includedon an integrated circuit. The test circuit 404 includes a digital toanalog converter (DAC) circuit 412, an excitation circuit 414 and ameasurement circuit 416. To measure the internal impedance, a testsignal is generated using a digital to analog converter DAC circuit 412.Using excitation operational amplifier 424, the excitation circuit 414applies an excitation circuit generated using the test signal to thesensor circuit. A monitoring signal resulting from the excitation isused by the measurement circuit 416 to determine the internal impedance.For instance, an excitation signal of a predetermined voltage can beapplied to the sensor circuit and the resulting current signal can beused to determine the internal impedance. The DAC circuit 412 allowsexcitation signals of different frequency and magnitude to be generated,such as by controlling the DAC circuit 412 with a processor or othercontrol circuit (e.g., a waveform generator).

When the internal impedance to be measured is low, the voltage of theexcitation signal may need to be reduced to limit the current throughthe sensor and limit the power consumption of the test. A challenge withmeasuring internal impedances for different types of sensors with verydifferent internal impedances is that circuit noise can becomesignificant as the voltage of the excitation signal is reduced.

The excitation circuit 414 includes a configurable first circuit gainstage 420 and a configurable second circuit gain stage 418. The firstcircuit gain stage 420 includes a programmable gain amplifier (PGA). Thesecond circuit gain stage includes resistors Rd and cross coupled switchcircuit 422. The gain of the first circuit stage is configurable bychanging the programmable gain and the gain of the second circuit gainstage 418 is configurable by changing the state of the cross coupledswitch circuit 422. The combined signal gain of the excitation circuit414 is configured by the measurement circuit (e.g., using a controlcircuit) according to whether the internal impedance of the sensorcircuit to be measured is in a higher impedance range or lower impedancerange. A higher signal gain is provided when the internal impedance isin the higher impedance range.

When the sensor circuit has a value of internal impedance in the higherrange, the excitation circuit 414 is configured in a first gain mode.The excitation circuit generates a first excitation signal from the DACcircuit test signal using a first signal gain applied by the firstcircuit gain stage and a second signal gain applied by the secondcircuit gain stage. In an example not intended to be limiting, the gainof the first circuit gain stage 420 is 1 and the gain of the secondcircuit gain stage 418 is two to provide an overall signal gain of twoto the test signal in the first gain mode.

When the sensor circuit has a value of internal impedance in the lowerrange, the excitation circuit 414 is configured in a second gain mode.The excitation circuit generates a second excitation signal from the DACcircuit test signal using a third signal gain applied by the firstcircuit gain stage and a fourth signal gain applied by the secondcircuit gain stage. The gains in the second gain mode can be gain valuesbetween zero and one. In an example, the gain of the first circuit gainstage 420 is one-tenth ( 1/10) and the gain of the second circuit gainstage 418 is one-half (½) to provide an overall signal gain ofone-twentieth ( 1/20) to the test signal in the second gain mode. Thevalues of signal gain provided by the second gain mode can be changed bychanging the resistance values. In certain examples, the second gainstage provides a gain of four or five in the first gain mode andone-fourth (¼) or one-fifth (⅕) in the second gain mode. The measurementcircuit 416 selectively initiates application of the first excitationsignal or the second excitation signal to the electronic sensor andcalculate the internal impedance of the sensor. The small signal gainwhen the internal impedance is in the lower range reduces circuit noiseof the PGA and excitation operational amplifier 424. This improvesaccuracy in the internal impedance measurement in the lower range.

FIG. 5 is a flow diagram of an example of a method 500 of controlling atest circuit to measure an internal impedance of an electronic sensor.At 505, a first excitation signal in a first gain mode is applied to thesensor when the internal impedance of the sensor has a first internalimpedance range. In the first gain mode, the first excitation signal isgenerated from a test signal using a first signal gain generated using afirst circuit gain stage and a second signal gain generated using asecond circuit gain stage.

At 510, a second excitation signal in a second gain mode is applied tothe sensor when the internal impedance of the sensor has a secondinternal impedance range. In some examples, the values of impedance inthe second impedance range are lower than the value of impedance in thefirst impedance range. In the second gain mode, the second excitationsignal is generated from the test signal using a third signal gain usingthe first circuit gain stage and a fourth signal gain using the secondsignal gain stage.

At 515, the internal impedance of the sensor is calculated using thefirst excitation signal when in the first gain mode and using the secondexcitation signal when in the second gain mode. The calculated internalimpedance can be provided to a user or process. The internal impedancerange of the sensor circuit and consequently which signal gain to use inmeasuring the sensor circuit may be known ahead of time (e.g., by thetype of sensor) and programmed by a user. In other examples, themeasurement circuit can determine which signal gain to apply to theexcitation circuit. In some examples, the sensor circuit includes anidentifier (e.g., a machine-readable type code) read by the testcircuit. In some examples, an excitation signal of a predeterminedvoltage is applied to the sensor circuit to determine a coarse impedancemeasurement to determine the internal impedance range of the sensorcircuit and the signal gain is set accordingly.

FIG. 6 is circuit diagram of portions of another example of a testcircuit 604 electrically coupled to a sensor circuit 606. The testcircuit can be included in an integrated circuit. In certain examples,the sensor circuit is included in the same integrated circuit and thetest circuit. The electronic sensor can be an electrochemical sensor ora gas sensor. The impedance of the electronic sensor may change as thesensor is used and measuring the impedance of sensor can be useful toestimate the remaining useful life of the sensor. Determining when thesensor is near the end of its useful life may be more important if thesensor is used to sense a toxic substance or gas.

The test circuit 604 includes an excitation circuit 614 and ameasurement circuit 616. The excitation circuit provides an excitationsignal for measuring impedance. The measurement circuit can includelogic circuitry to perform the functions described and may includecontrol circuitry to initiate the measurements and calculations. Todetermine an unknown impedance of a sensor circuit Rx, an excitationsignal Vexc can be applied to the sensor circuit and the sensor currentIx can be measured, where Ix=Vexc/Rx. The same excitation signal Vexc isapplied to a known calibration resistance 626 (Real) and the calibrationcurrent Ical is measured, where Ical=Vexc/Rcal. Because Vexc is the samein both measurements, the unknown impedance of the sensor can bedetermined as Rx=Rcal(Ical/Ix).

This measurement approach works well if the value of Rx is the sameorder of magnitude as Rcal. However, the approach works less well if thesensor impedance is too different from Rcal. For instance, if Rcal isabout 200Ω and Vexc is 1V, then Iexc is about five milliamps (5 mA). IfRx is one megaohm (1 MΩ), then the Ix is about one microamp (1 μA).These values of current are too different for the measurement circuitryto provide the desired accuracy. Adding gain to the signal formeasurement may also add gain to the signal error, which will increaseerror in the measurement of the signal.

The test circuit 604 of FIG. 6 includes an adjustable bridge resistance628 in addition to the calibration resistance. The value of the bridgeresistance is adjusted to be between the resistance value of Rcal andthe value of the sensor impedance. The bridge resistance can be aninexpensive resistance circuit and the resistance may not be known tothe desired accuracy. The test circuit 604 can include a multiplexercircuit that can be used to selectively apply the excitation signal tothe calibration resistance, the adjustable bridge resistance, or thesensor circuit 606.

A first predetermined excitation signal can be applied to Rcal and thebridge resistance Rb, and a second predetermined excitation signal canbe applied to the bridge resistance Rb and the sensor circuit Rx.Different signal gain can be applied to the separate excitation signalsand the measured currents can be used to determine the impedance of theelectronic sensor.

For instance, the measurement circuit 616 may apply a first excitationsignal Vexc₁ to the calibration resistance Rcal and measure acalibration current Ical. The first excitation signal is applied to thebridge resistance Rb and a first bridge current Ib₁ can be measured. Themeasurement circuit 616 applies a second excitation signal Vexc₂ to thebridge resistance value and a second bridge current Ib₂ can be measured.The second excitation signal is applied to the sensor and a sensorcurrent Ix is measured. The impedance of the sensor is determined asRx=(Rcal)(Ical/Ib ₁)(Ib ₂ /Ix),where Ib₁ is the current in the bridge resistance when the excitation isVexc₁ and Ib₂ is the current in the bridge resistance when theexcitation is Vexc₂. Because the measurement excitation voltage and gainare the same for each pair of currents measurements, the accuracy of themeasurement voltage and the gain in the measurement system isunimportant.

In an example intended to be illustrative and non-limiting, if thesensor impedance is known to be in the range of about 200Ω, and Rcal is10 kΩ, the bridge resistance can be 1.128 kΩ. In this case the ratio ofthe bridge resistance and the calibration resistance, and the ratio ofthe bridge resistance and the sensor impedance may not be too large, andone value of bridge resistance can be used. The measurement circuit mayapply an excitation signal to the sensor to get an estimate of theimpedance before setting the value of the bridge resistance. Themeasurement is an estimate because the excitation signal typicallydoesn't have the required accuracy. The measurement circuit 616 may setthe bridge resistance value to a coarse or approximate resistance valueprior to more accurately determining the bridge resistance value. Themeasurement circuit 616 can also be used to measure the resistance ofRcal if desired. The multiplexer circuit 630 can be used to apply theexcitation signal to the calibration resistance and the measurementcircuit 616 can calculate the calibration resistance using theexcitation signal.

In another example, if the impedance difference between the sensorimpedance Rx and the calibration resistance Rcal is too large, multiplebridge resistance steps can be used to bridge the measurements betweenRcal and Rx. For instance, the measurement circuit 616 may apply a firstexcitation signal Vexc₁ to the calibration and the first bridgeresistance Rb₁ to measure the calibration current Ical and the firstbridge current Ib₁₁ as in the previous example, where Ib₁₁ is the bridgecurrent for the first bridge resistance value and the first excitationsignal. The first bridge resistance value may be set closer to Rcal thanRx. The measurement circuit 616 applies a second excitation signal tothe first bridge resistance value and a second bridge current Ib₁₂ ismeasured, where Ib₁₂ is the bridge current for the first bridgeresistance value Rb₁ and the second excitation signal Vexc₂.

The bridge resistance is then changed to a second value Rb₂ that may becloser to Rx than Rcal. In an example intended to be illustrative andnon-limiting, if the sensor impedance is known to be in the range ofabout 200Ω, and Rcal is 10 kΩ, the value of Rb₁ may be set to 712Ω andRb₂ may be 2.53 kΩ. The second excitation signal Vexc₂ is applied to thesecond bridge resistance and a third bridge current Ib₂₂ is measured. Athird excitation signal Vexc3 is applied to the second bridge resistanceRb₂ and a fourth bridge current Ib₂₃ is measured, where Ib₂₃ is thebridge current for the second bridge resistance value Rb₂ and the thirdexcitation signal Vexc₃. The third excitation signal is then applied tothe sensor impedance Rx and the sensor current Ix is measured. Theimpedance of the sensor Rx can then be determined using the calibrationresistance Rcal, the calibration current Ical, the first bridge currentIb₁₁, the second bridge current Ib₁₂, the third bridge current Ib₂₂, thefourth bridge current Ib₂₃, and the sensor current Ix, byRx=(Rcal)(Ical/Ib ₁₁)(Ib ₁₂ /Ib ₂₂)(Ib ₂₃ /Ix).

FIG. 7 is a flow diagram of an example of a method 700 of controlling atest circuit to measure an internal impedance of an electronic sensor.At 705, a first predetermined excitation signal is applied to acalibration resistance and a calibration current is measured. A bridgeresistance value is then selected. In some examples, an approximate orcoarse value of the sensor impedance is determined by the test circuitand the bridge resistance value is set accordingly by the test circuitor a user. The test circuit may include a table stored in memory, andthe test circuit may set the bridge resistance by using the approximatesensor impedance as an index into the table. At 710, the same firstexcitation signal is applied to the bridge resistance and a first bridgecurrent is measured.

At 715, a second predetermined excitation signal is applied to thebridge resistance and a second bridge current is measured. At 720, thesame second excitation signal is applied to the sensor and a sensorcurrent is measured. At 725, the internal impedance of the sensor iscalculated by the test circuit using the calibration resistance, thecalibration current, the first bridge current, the second bridgecurrent, and the sensor current. The calculated impedance may then be toa user or process. For example, the internal impedance may be used togauge the remaining useful life of the electronic sensor.

More than one bridge resistance may be needed. The bridge resistance maybe determined according to a ratio. The test circuit may determine thebridge resistance to keep a ratio of the calibration impedance and thebridge impedance to about four. If the ratio of the selected bridgeimpedance and the approximate sensor value is not also within a desiredratio, the test circuit may select a second bridge resistance that iswithin the desired ratio of the sensor and within a desired ratio of thefirst bridge resistance value. The sensor impedance is then determinedusing the calibration resistance, the calibration current, four bridgecurrents and the sensor current as in the example described previously.This approach can be extended. More than two bridge resistances may beneeded if the difference in values of the calibration resistance and thesensor impedance is too great.

FIG. 8 is a circuit diagram of portions of another example of a testcircuit 804 electrically coupled to a sensor circuit 806. In addition toaddressing power consumption and circuit noise, circuits to monitor theoutput of electronic sensors may need to tailor the output of the sensorcircuits to improve accuracy.

The test circuit 804 can be included on an integrated circuit andincludes an input to receive an electrical signal from the sensorcircuit 806, a DAC circuit 832, a PGA 834, and measurement circuit 836.In certain examples, the sensor circuit is included on the sameintegrated circuit and the test circuit. The sensor can be used todetect a concentration of a chemical or gas within the environmentcontaining the sensor. The internal impedance of the sensor varies withthe concentration. The sensor is coupled to a resistive divider thatincludes a reference resistor 840 (Rref). A switch 842 may enable theresistive divider during measurement of the sensor output. The switchmay also be activated for a short time (e.g., 200 microseconds (200 μs))to enable the current path through the sensor to briefly heat the sensorand then deactivated to save the power. By measuring Rref, the currentcan be monitored to precisely control the average power burning on thesensor, to precisely control the temperature to precisely control thesensitivity of the sensor for the measurement.

The measurement circuit 836 may include a controller to open the switchto conserve energy when a measurement is not performed. The voltage Vccapplied to the sensor is divided by the internal impedance of the sensor806 and the reference resistor 840. To monitor the sensor, the voltageof the signal from the sensor varies with the concentration of thechemical or gas. For instance, the sensor may be a gas sensor. Theinternal impedance of the sensor may vary with the gas concentration andprovide a voltage proportional to a concentration of the gas.

The signal from the sensor circuit 806 includes a direct current (DC)offset and a varying signal component. The DC offset can occur when theinternal impedance of the sensor circuit has an approximate impedancethat varies by a small amount. For example, the internal impedance ofthe sensor may vary from between 18Ω-20Ω over the range of theconcentration of the gas. The voltage due to the 18Ω would appear as aDC offset in the electrical signal from the sensor.

FIG. 9 are graphs of an example of the output of the sensor circuit 806as a function of the concentration of a gas. The DC component is shownas Vshift. Graph 905 shows that the voltage output varies between 2.37Volts (2.37V) and 2.5V. The DC offset prevents using a lower voltagemeasurement circuit if desired and can limit the measurable range of thechange in the output, which can impact accuracy of the measurement. Toimprove accuracy, the DC offset is removed. Graph 910 shows the outputof the sensor after shifting the output to be centered on zero volts andsignal gain applied to the signal from the sensor. The graph 910 showsthat the accuracy of the measurement can be improved by the largersignal range, and a measurement circuit with a lower voltage can be usedto monitor the output.

The test circuit 804 includes a DAC circuit 832 to remove the DC offsetin the signal received from the sensor. The DAC circuit 832 can beprogrammed be user to subtract a known DC offset or automaticallyadjusted by a control circuit to subtract a measured DC offset. The PGA834 applies signal gain to the signal shifted by the DAC circuit. Theamount of gain provided by the PGA 834 may also be set by a user orautomatically adjusted using a control circuit. The measurement circuit836 generates a measure of the varying signal component. In someembodiments, the measurement circuit 836 includes an analog-to-digitalconverter (ADC) circuit 838 to generate a digital value representativeof the signal form the sensor circuit 806. In the example of FIG. 8 ,the ADC circuit is a sixteen bit ADC circuit. The signal gain providedby the PGA allows for more signal swing that makes the accuracy of the16-bit ADC more useful. Depending on the application, the measurementcircuit may also include a fast Fourier transform (FFT) circuitconfigured to measure a frequency response of the varying signalcomponent.

FIG. 10 is a flow diagram of an example of a method 1000 of controllinga test circuit to measure an internal impedance of an electronic sensor.At 1005, an input signal is received by the test circuit from theelectronic sensor. The input signal includes a DC offset component and avarying signal component. At 1010, the DC offset is removed from theinput signal using a DAC circuit and signal gain is applied to theremaining the varying signal component. At 1015, the varying signalcomponent with the applied signal gain is measured using a measurementcircuit. The measure can include a digital value determined using an ADCcircuit. The measurement can be provided to one or both of a user orprocess. For instance, the sensor may be an oxygen sensor. As shown inFIG. 8 , the test circuit can include a detection circuit 844. Thedetection circuit may generate an indication of a lower explosive limit(LEL) according to the measure of the varying signal component. Theindication may be used to generate an alert regarding the LEL.

The devices, methods, and systems described herein allow for monitoringsmart sensors with low power consumption, improved accuracy.

ADDITIONAL DESCRIPTION

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” All publications, patents, and patent documentsreferred to in this document are incorporated by reference herein intheir entirety, as though individually incorporated by reference. In theevent of inconsistent usages between this document and those documentsso incorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein.” Also, in the following claims, theterms “including” and “comprising” are open-ended, that is, a system,device, article, or process that includes elements in addition to thoselisted after such a term in a claim are still deemed to fall within thescope of that claim. Moreover, in the following claims, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects. Methodexamples described herein can be machine or computer-implemented atleast in part.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A test circuit for an electronic sensor, the testcircuit comprising: a load resistance connectable in series with theelectronic sensor to form a series resistance of the load resistance andan internal impedance of the electronic sensor; an excitation circuitconfigured to apply a predetermined voltage to a circuit element; and ameasurement circuit configured to: initiate applying the predeterminedvoltage to the series resistance and determining the series resistance;initiate applying the predetermined voltage to the load resistance anddetermining the load resistance; and calculate the internal impedance ofthe electronic sensor using a difference of the determined seriesresistance and the load resistance, and provide the calculated internalimpedance to a user or process.
 2. The test circuit of claim 1, whereinthe excitation circuit is configured to apply a specified electricalsignal having a signal amplitude less than twenty millivolts (20 m V) tothe series resistance, and the internal impedance of the electronicsensor is less than ten ohms (10Ω).
 3. The test circuit of claim 1,wherein the electronic sensor is an electro-chemical sensor.
 4. The testcircuit of claim 1, wherein the measurement circuit is configured tocalibrate the predetermined voltage applied using the excitationcircuit.
 5. The test circuit of claim 1, wherein the test circuit isconfigured to: connect a calibration resistor of a predeterminedresistance to an output of the test circuit; adjust an excitation signalapplied by the excitation circuit until the test circuit measures aspecified current corresponding to the predetermined voltage; and applythe predetermined voltage to the series resistance and the loadresistance.
 6. A method of monitoring an electronic sensor using a testcircuit, the method comprising: connecting a load resistance in serieswith the electronic sensor to form a series resistance of the loadresistance and an internal impedance of the electronic sensor; applying,using a measurement circuit, a predetermined voltage to the seriesresistance and determining the series resistance; applying thepredetermined voltage to the load resistance and determining the loadresistance; calculating the internal impedance of the electronic sensorusing a difference of the determined series resistance and the loadresistance; and providing the calculated internal impedance to a user orprocess.
 7. The method of claim 6, wherein the applying thepredetermined voltage includes applying a specified electrical signalhaving a signal amplitude less than twenty millivolts (20 m V) to theseries resistance; and wherein calculating the internal impedance of theelectronic sensor includes calculating an internal impedance less thanten ohms (10Ω).
 8. The method of claim 6, wherein calculating theinternal impedance of the electronic sensor includes calculating theinternal impedance of an electro-chemical sensor.
 9. The method of claim6, including calibrating the predetermined voltage.
 10. The method ofclaim 9, wherein calibrating the predetermined voltage includes:electrically connecting, by the test circuit, a calibration resistor ofa predetermined resistance to outputs of the test circuit; adjusting, bythe test circuit, an excitation signal until a specified current ismeasured corresponding to the predetermined voltage of the excitationsignal; and applying the predetermined voltage to the series resistanceand the load resistance.
 11. An integrated circuit comprising: a sensorcircuit having an impedance; a load resistor connectable in series withthe sensor circuit to form a series resistance of the load resistor andthe internal impedance of the sensor circuit; an excitation circuitconfigured to apply a predetermined voltage to a circuit element; and ameasurement circuit configured to: initiate applying the predeterminedvoltage to the series resistance and determining the series resistance;initiate applying the predetermined voltage to the load resistor anddetermining load resistance of the load resistor; and calculate theimpedance of the sensor circuit using a difference of the determinedseries resistance and the load resistance.
 12. The integrated circuit ofclaim 11, wherein the excitation circuit is configured to apply aspecified electrical signal having a signal amplitude less than twentymillivolts (20 m V) to the series resistance, and the impedance of thesensor circuit is less than ten ohms (10Ω).
 13. The integrated circuitof claim 11, wherein the sensor circuit includes an electro-chemicalsensor.
 14. The integrated circuit of claim 11, wherein the measurementcircuit is configured to calibrate the predetermined voltage appliedusing the excitation circuit.
 15. The integrated circuit of claim 11,including: a calibration resistor; wherein the excitation circuit isconfigured to apply an excitation signal to the calibration resistor;wherein the measurement circuit is configured to: adjust the excitationsignal applied to the calibration resistor until a specified currentcorresponding to the predetermined voltage is measured; and initiateapplying the predetermined voltage to the series resistance and the loadresistance.